1. Field of the Invention
The present invention demonstrates the method of wavelength measurement and tracking by using a semiconductor laser amplifier.
2. Description of the Prior Art
With the flourishing development of computer and the rapid growth of internet applications, people have higher demand on the channel bandwidth for data transmission. Since optical fiber has advantages of wide bandwidth and low loss, it has become the main medium of network transmission. In response to the rapid growth of the channel capacity in communication networks, ITU has established the standard for multi-wavelength (frequency) optical networks, and various wavelength multiplexing devices and technology are developed rapidly. The key point of these devices and technology lies in the control and stabilization of the wavelength of lasers. Therefore, an economic and effective method of wavelength control and stabilization will make a significant contribution to the development of optical networks.
So far, single frequency lasers and tunable wavelength lasers for high speed optical networks and telecommunications have already been mass produced. With the reduction of cost, the application of these laser sources is growing very fast. Since wavelength is the critical resource of an optical fiber network, its measurement and control becomes an important issue. The error in wavelength display or measurement of electro-optic instruments is typically about 0.5 nm. However, the channel spacing of WDM networks is as small as 0.8 nm, so the accuracy of laser wavelength and its stability must be on the order of 0.1 nm or better.
The current technology for the applications of the following devices is described below:
(1) wavelength measurement or sensing: an electro-optic mechanism which is wavelength sensitive should be used in measuring the wavelength. Interferometry is the most frequently used mechanism for wavelength measurement, e.g., in U.S. Pat. No. 5,392,120 and U.S. Pat. No. 5,493,395, interferometers are used as the main component in wavelength measurement, in which the wavelength dependency of the combined signal formed by the optical path length difference between different routes or multiple reflections was utilized to distinguish the wavelength. In general, the accuracy of wavelength measuring instrument can be as high as 1 ppm. Adding a reference laser beam with stable and accurate wavelength can greatly increase the measurement accuracy. Besides, T. Coroy et al. mentioned the method of wavelength measurement using the photodetector with optical filter in IEEE photonics technology Letters (vol. 8, no. 12, pp. 1686-1688, 1996). PA1 (2) Wavelength tracking and wavelength stabilization: Interferometers with special feedback control circuit for wavelength measurement are usually used to match a special position in the output characteristics of the interferometer (e.g. its peak value) with the wavelength of incident light. To stabilize the wavelength, the feedback control circuit and reference beam with stable and sensitive wavelength characteristics is used. Several frequently used methods are described in the book "Frequency Stabilization of Semiconductor Laser Diodes" written by Ikegami et al. (1995) The components or approach to generate or serve as the reference wavelength are: PA1 (3) Wavelength comparison: few methods are proposed in this respect, so far only M. Guy et al. mentioned a wavelength comparator by applying the nonlinear effect of optical waveguide with photodetector array in Journal of Lightwave Technology (Vol. 14, No.6, pp. 1136-1143, 1996 ), in which the diffraction angle varies with different wavelength and thus the wavelength of the incident light can be compared.
(i) Interferometer with fixed wavelength characteristics. PA2 (ii) Optical fiber grating with sharp transmission spectrum. PA2 (iii) Specific spectral lines (e.g. Na, Rb, Cs, etc.) from the energy level transition of metal atoms. PA2 (iv) Optogalvanic effect: Certain gas lamp will change its electric property under the light illumination at some specific wavelength (e.g. Kr, at 1.524380 ?m). PA2 v) Gas unit with a specific absorption wavelength.
Additional objects, advantages and novel features of the invention will be set forth in part in the description as follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.